On: OpenBSD is so fast, I had to modify the program slightly to measure itself

[loeg]

FreeBSD doesn't use RCU in the kernel, so it is unlikely to have the exact same ~bug as Linux here. Any naive implementation will have the same decent performance OpenBSD does for this program.

FreeBSD's equivalent implementation lives here (it does all of this under an exclusive lock): http://fxr.watson.org/fxr/source/kern/kern_descrip.c#L1882

It doesn't use RCU, but does do a kind of bespoke refcounted retention of old versions of the fdtable on a free list.